Rad drain tap hole thread

[Bob Culver]

I have found it more convenient to count hthreads in say 1/4 inch using the ever handy vernier gauge. 
Probably not applicable here but poor threads can sometimes be improved by sawing slots in a bolt, preferably tough stuff, as a tap.
From various sources charts are availablae of threads in progressive sizes with tpi/mm pitch, tap sizes etc. Very helpful for identity and for figuring remedies.